Session Item

The electron IntraOperative Radiation Therapy (IORT) consists in delivering an high dose to the target during surgery. The aim of this work is to highlight geometric and dosimetric critical issues of IORT technique and to report practical solutions, based on the expertise gained from clinical practice with mobile linacs, for about one thousand patients.

The main geometric issue concerns the matching between the treatment and the beam characterization set-ups. This matching is mandatory for a correct dosimetric evaluation. In our approach, a circular PMMA disc (2 cm larger than diameter of applicator) is put on the target surface so that the applicator, giving a lightly pressure on the surface, homogenizes the target volume and ensures a useful buildup effect. The thickness of the disc is optimized according to the target thickness and the beam energy. To ensure the stability of the treatment set-up and the alignment applicator-target-internal shield, a flat (or little sloped) applicator vertically oriented is used.
Dosimetric issues mostly concern beam and detector characterization, choice of treatment energy, knowledge of backscattered dose inside the target, output reproducibility and implementation of in-vivo dosimetry. The use of a disc on the target surface ensures a useful buildup effect allowing to use the highest available beam energy. Regarding the output reproducibility, a conditioning modality is implemented and systematic treatment simulations performed to check the output with a timing similar to that of the clinical practice. Moreover, since the applicator shape strongly affects the beam characteristics attention is paid to check the applicator wholeness after the washing and sterilization process (fig 1: effects due to a longitudinal applicator warp caused by an erroneous process with high temperature). For in-vivo dosimetry, the detector correct positioning is ensured by firmly fixing the detector to the disc by a steril-strip.

The disc on the target surface allows to avoid herniation of the target inside the applicator, which could cause a significant increase in the delivered dose (about 5% for 1 cm of homogeneous herniation) and an unsuitable dose distribution to the target. Moreover, using the highest available energy and optimizing the disc thickness allow a better dosimetric coverage of target (entrance dose at least 95%, dose increase in the pre-buildup region up to 6%).
Sometimes a damaged applicator can be delivered after sterilization then a careful check excludes reuse of the damaged applicator.
A micromosfet detector sandwiched between the applicator and the target surface permits to set the intervention level for in-vivo dosimetry at 5%.

The described treatment set-up ensures a good coupling between applicator and target surface and permits to take advantage of all the dosimetric evaluations obtained in water and slab phantoms. Furthermore, the implementation of in-vivo dosimetry ensures an independent dose assessment.